The disclosed subject matter relates generally to very high power laser systems, such as gas discharge lasers, and more particularly, to methods and systems for improving optical components in a gas discharge laser chamber.
Electric discharge gas lasers are well known for utilization in such fields as integrated circuit photolithography manufacturing processes. The advent of immersion photolithography has required manufacturers of such laser systems to offer lasers that can produce 60 to 90 Watts of average power and more, meaning the laser light source needs to produce output light pulses, by way of example with 20 mJ or more of pulse energy at repetition rates of 4 kHz or 15 mJ of output pulse energy at 6 kHz, the former resulting in an 80 Watt laser and the latter in a 90 Watt laser.
Excimer lasers are one type of electric discharge gas laser. Excimer lasers have been known since the mid 1970s. A description of an excimer laser, useful for integrated circuit lithography, is described in U.S. Pat. No. 5,023,884 issued Jun. 11, 1991 entitled “Compact Excimer Laser.” The '884 patent has been assigned to Assignee of the present application. The '884 patent is hereby incorporated herein by reference for all purposes. The excimer laser described in '884 patent is a high repetition rate pulse laser, though the laser disclosed had an output pulse repetition rate of about one third to one half that of contemporary laser systems.
To produce such pulse energies at such pulse repetition, it has been suggested in the above referenced co-pending patent applications to use a master oscillator/power oscillator configuration, especially in applications where the seed laser (master oscillator (“MO”)) is utilized to finely tune such parameters as center wavelength and beam quality parameters like bandwidth, at relatively low output pulse energies, and the amplifier portion (power oscillator (“PO”) or power ring amplifier (“PRA), which is also an oscillator) then amplifies the seed laser output pulses to attain the 15-20 mJ or so laser system output pulses. Throughout this application for the sake of convenience of description the laser system may be referred to as master oscillator/power ring amplifier (“MOPRA”) or a master oscillator/power oscillator (“MOPO”), which a MOPRA also is, or a master oscillator/power amplifier (“MOPA”). However, for purposes of this application and the meaning of the  accompanying claims, these terms are intended to be interchangeable, and further are intended to include within the relevant disclosure high poser laser systems that are not of the seed laser/amplifier laser system variety, unless expressly so stated otherwise, i.e., that any aspect of the disclosed subject matter is limited in application to only a certain one or more of such laser arrangements. That is, regardless of the laser arrangement involved, similar components similarly situated, configured and utilized and facing similar detrimental optical influences from short term and/or long term exposure to such high energy light can employ any aspect(s) of an embodiment of the claimed subject matter.
It will also be understood that some or all of the problems faced in producing such very high power laser system output pulses may also be faced in single or dual laser system producing such very high power outputs, such as broad band lasers used for such applications as laser annealing to form crystallized semiconductor material (e.g., silicon) for thin film transistor and the like manufacturing on flat panels and the like. The disclosure and the meaning of the accompanying claims, unless otherwise so expressly limited are not meant to exclude such laser arrangements.
Utilization of an oscillator as the amplification laser mechanism results in certain operating problems which increase in severity as output pulse repetition rate increases, as they are mostly optical fluence and optical thermal transient induced problems. An oscillator by its very nature, bounded by two mirrors defining the cavity, one of which must be partially reflective to allow useful light, generated by the lasing in the cavity, to leave the cavity, must generate more energy in each laser pulse than leaves the laser cavity as useful light. The difference between the energy circulating within the cavity and that leaving the cavity depends on a number of factors such as cavity geometry, the reflectivity of the partially reflective mirror (referred to as the output coupler (“OC”)). However, as an example, to generate output pulse energy of 15 mJ, the cavity may see closer to 20 mJ or more. A similar relation exists to the energy in the cavity when the output pulse energy is even higher, such as 20 mJ. Thus generating very high average power outputs with such lasers, e.g., 60-100 W, as currently required for immersion lithography, can put the optics within the cavity under very high fluence loads, resulting in, among other things high thermal stress and transients.
Similar, though perhaps less severe, effects may be seen with broad band seed laser/amplifier laser arrangements, where the seed laser input pulse energy to the amplifier laser is higher (perhaps by an order of magnitude) and is amplified, whether or not the amplifier is also an oscillator or not. Likewise even the optics in a single chamber laser system may experience high loading that can benefit from aspects of an embodiment of the disclosed subject matter. Thus, applicants propose measures to remove or at least reduce the impacts of such high optical fluence in a laser cavity, whether the cavity forms a single chamber laser or is the amplifier in a seed laser/amplifier laser system or in amplifier lasers whether such include an oscillation cavity or simply utilize a fixed, optically defined, number of passes through the amplifier laser gain medium.
Unfortunately many of the optical components exposed to such very high fluence/power are not optimized for the high power laser beams due to budgetary constraints, manufacturing difficulties or simply the availability of materials with suitable properties to sustain such fluence/power and/or long term exposure to such fluence/power. When the optical components are not optimized, the less than optimum components can absorb a portion of the laser rather than pass the laser beam through the optical component, transmissively or by total internal reflection. Absorbing a portion of the laser can cause optical component to increase in temperature and the increase in temperature can cause the optical component to distort and misdirect the light which degrades the performance and otherwise reduces the output power of the laser or degrade laser beam characteristics such as beam stability and polarization. What is needed is a more cost-effective method of making and/or utilizing optical components included in the high-power laser light path.